Multi-layer printed circuit boards and methods of making same

ABSTRACT

A MULTI-LAYER PRINTED CIRCUIT BOARD FOR USE IN MICROWAVE FREQUENCY CIRCUITS IS DESCRIBED. THE BOARD HAS AT LEAST A PAIR OF PRINTED CIRCUIT BOARDS HAVING POLYTETRAFLUORETHYLENE SUBSTRATES, ONE OF WHICH IS SINGLE SIDED AND THE OTHER OF WHICH IS DOUBLE SIDED. ONE CIRCUIT OF THE DOUBLE SIDED BOARD PROVIDES AN INTERNAL LAYER OF THE MULTI-LAYER BOARD. THE OUTER LAYERS FORM GROUND PLANES SUCH THAT STRIPLINE   CIRCUITS ARE PROVIDED FOR CONNECTION TO MICROWAVE CIRCUIT COMPONENTS. THE PROCESS INCLUDES STEPS FOR LAMINATING THE BOARDS TOGETHER PROVIDING PLATED THROUGH HOLES WHICH PERMIT POLYETRAFLUORETHYLENE TO BE USED IN A MANNER COMPATIBLE WITH MICROWAVE CIRCUIT REQUIREMENTS.

March 14, 1972 J, F, DEGNAN ET AL 3,649,475

MULTI-LAYER PRINTED CIRCUIT BOARDS AND METHODS OF MAKING SAME Filed July22, 1969 MULTI-LAYER f PRlNTEDfCIRCUITA BOARDS AND '.x METHODS OF MAKINGSAME Joseph F. Degnan, Webster, and Robert A. Ayotte,

Rochester, N.Y., assignors to General Dynamics Corporation y j, i iFiled July 22, 1969, Ser. No. 843,435

U.S. (31.204-15 i 7 Claims ABSTRACT' OF THE DISCLOSURE lA multi-layerprinted circuit board for use in microwave frequency circuits isdescribed. The board has at least a pair of printed circuit boardshaving polytetrafluorethylene substrates, one of which is single sidedand the other of which is doublev sided'. One circuit of the doublesided board provides an internal layer of the multi-layer board. Theouter layers form ground planes such that stripline circuits areprovided for connection to microwave circuit components. The processincludes steps for laminating the boards together providing platedthrough holes which permit polytetrauorethylene to be used in a mannercompatible with microwave circuit requirements.

The present invention relates to multi-layer printed circuit boards andmethods of making same, and particularly to microwave or high frequencyprinted circuit boards having stripline circuits and methods of makingsuch boards.

' yThe invention is especially suitable for use in providing a circuitboard wherein the circuits are etched on polytetrauorethylene substratesto a high degree of precision so as to provide the electricalcharacteristics necessary for stripline circuits. Features of theinvention, however, are applicable for manufacturing of multi-layerboards having precise spacing and void free laminations, as may be'required with other substrate materials.

' .,Polytetrauorethylene, better known by the trade name Teon which hasbeen adopted by the E. I. du Pont de Nemours and Company, Inc. ofWilmington, Del., is a desirable substrate for stripline circuits. Thetrade name .Teflon will be used hereinafter to designate this material.Unfortunately, it has not heretofore been possible to laminatesuccessive layers of Teflon together in a manner to provide multi-layerboards of requisite precision for stripline circuitry. The adhesives,often chemically dissimilar to the substrate, which have been utilizeddid not provide a reliable bond or were incompatible with materialsutilized to provide plated through holes, resulting in inferior holeplating. Accordingly, staked connections, say using rivets, wereemployed, together with other means for providing 'an additionalmechanism for holding the boards together and to obtain shielding.Previous boards were therefore unsuitable for handling high voltages,since corona occurred in voids left during the lamination process'.Another `deiiciency of prior boards was non-uniform spacing between thecircuit planes. Thus, the yield rate of processes for manufacturing suchboards was very low and their cost high.

It is an object of the present invention to provide improved multi-layerboards and methods of making same wherein the foregoing disadvantagesand difficulties are substantially eliminated.

It isa still further object of the present invention to provide improvedmulti-layer printed circuit boards which may be Yconstructed fromcircuits printed on Teflon substrates.

It is a still further object of theA present inventionto Patented Mar.14, 1372 ice It is ar still further'ob'ject of the'presentyinvent'i'onto provide methods of making multi-layer printed circuitboards wherein the circuit layers are precisely spaced and registeredwith respect to each other. i,

It is a still further object of the present invention to provide methodsof making multi-layer printed circuit boards having Tellon layerstogether with plated through holes.

It is a still further object of the present invention to provide amethod of making multi-layer printed circuit boards with Teilonlaminations which have void free monolithic construction.

It is a still further object of the present invention to provide amethod of making multilayer printed circuit boards having ,Teflonlaminations which withstand vibration even over a wide range oftemperatures.

A still further object of the invention is to provide an improvedtechnique for drilling holes through a multi-layer board which insuresthat the rings of conductive material in the holes are clean, therebyeliminating the need for removal of materials from other layers whichmay smear over the rings and especially for the removal of Teon forwhich no suitable chemical removal technique is available.

A still further object of the invention is to provide an improvedtechnique for preparing dissimilar surfaces, such as the adjacent layersof a through-hole in a multi-layer printed circuit board, for plating,so that the surfaces can be uniformly plated in a single process.

Brieiiy described, a multi-layer printed circuit board embodying theinvention includes at least two printed circuit boards, one of which issingle sided and the other which is double sided. The boards havesubstrates of Teon. Both boards are initially copper cladded. The coppercladding is etched off the internal surface of one of the boards whichis to become an inner layer of the multi-layer structure. A sheet ofnon-flowing B stage epoxy glass cloth is disposed between the innersurafces of the board and bonds the individual boards together uponlamination into a monolithic, void free, multi-layer structure. A pairof sheets, one of which is vinyl and the other a vinyl phenolic glasscloth material, may be disposed on the outer surfaces and laminatedtogether with the structure. After lamination, holes are drilled throughthe structure. These holes are later plated to provide plated throughhole connections between the circuits. The vinyl sheet laminates providestructural strength and prevent burring, crazing, or tearing duringdrilling, thereby helping to provide clean, internal rings. These masksalso prevent undesirable plating or etching on exposed Teflon surfaces.

The Teflon boards and the epoxy glass cloth are laminated together viameans which equalizes the pressure applied thereto, providing veryprecise spacing and precludingrnis-registration of the circuit layers-due to differential shear stresses which may occur during lamination.Bond strength is maximized through uniform glass-resin ratio retention,which insures uniformity 'of bonding film thickness, and providescontrollable resin flow. Drilling of the plated through holes isaccomplished with the aid of a cleaning material so as to preventcrazing or smearing of the Teflon over the printed circuit or epoxymaterial. After drilling, the holes are etched in two steps: firstly totreat the Teflon (viz. to coat it in a manner to accept deposit of aconductive coating), and secondly, to prevent any'epoxy smear oninternal copper rings which might reduce continuity of the printedcircuit with the plating provide 1. improvedr stripline. multi-layerYprinted-` circuit to be placed in the hole. After etch back, theconductive coating, say electroless copper, is deposited. The'board thenis plated to provide the plated through holes, and if required,.additional circuitry is printed on `thel exposed outer area of thecircuitside board.- f 1.,. v.

steps, such that the boards are completed and provide stripline circuitshaving plated through holes to which microwave components such.y asmicrowave tubes,` may be connected. ff. A'

. A evinvention itself,V thlstoits organization and method ofoperation'as-.well asadditional `objectsand advantages thereof, willbecomefmore readily apparent from the reading of the following`description in connection with the accompanying drawings in which:

FIG. 1 is a flow chart illustrating the steps in a process for makingmulti-layer printed circuit boards in accordance with the invention;

FIG. 2 is a sectional view illustrating a package of the individualboards and sheets which upon lamination forms the invention;

y FIG. 3 is a sectional view of a lamination fixture used to laminatethe package shown in FIG. 2;

FIG. 4 is a sectional view, the section being taken along the line 4-4of FIG. 5 showing a multi-layer printed circuit board embodying theinvention which is constructed in accordance with the process depictedin FIG. 1; and

FIG. 5 is a plan view of the board shown in FIG. 4.

Referring to FIG. 1, there is depicted the sequence of steps which isperformed in accordance with the invention for the fabrication ofmultilayer Tefion printed circuit boards consisting of two or morediscrete Teflon substrates with plated through holes as theinterconnection media. In the first step 100 of the process, the circuitboards and sheets which are to be laminated together are cut into blanksof suitable size. In the event that the circuit boards to be fabricatedare small, 'the blanks may be of sufficient size to accommodate severalboards, say 6 boards, one by two inches each per blank.

Alignment holes are drilled in the Teflon board which will receivealignment pins for assembly thereof into the fixture. These alignmentpins 15, four to a blank, are shown in FIG. 3. The index holes may bepunched in the other sheets which are to be laminated together with theboards. The blank Teflon boards are copper clad on both sides. Thethickness of the copper clad material may for example be about 0.31 in.Two Teflon boards are used to make each multi-layer board. The othermaterials used will be described in connection with the laminationpackages shown in FIG. 2.

In the next step 101, the boards are prepared, by which is meant thatcircuits patterns are provided thereon. In this course of boardpreparation, they are cleaned, photo resist is applied thereto, circuitsare printed thereon, developed, etched and the photo resist strippedwith a suitable solvent. The copper cladding is etched away from theentire surface of one of the boards which is to be disposed on theinside of the laminated multilayer board. It has been found inaccordance with this invention that, although Teflon may not belaminated with a dissimilar bonding material, such as epoxy glass, aTeon surface which has been clad with copper and from which the copperhas been etched away will accept lamination and bonds exceedingly wellwithout voids or wrinkles, when a bonding medium of substantiallynon-fiowing properties which retains high resin content, such asnon-flowing B stage vepoxy glass is used. l In the next step 102,cutouts such as windows'and holes are machined in the boards. Desirably,adrill is used for the holes and a drill mounted on a pantograph is usedto make the'cutouts. The laminating material (viz., the B stageepoxyglass cloth) is also provided with cutouts as-required. Desirably, thesecutouts should be cutl back slightly, say l@ in. from the'correspondingcutout areas inthegmating'cover board.

In thenext step-103, the lamination packages' 34 Vare assembled. FIG. 2shows one-of the packages 34. A number of these packages 34 can beassembled on the pins before the pins are inserted together with theassembled packages thereon into holes in a'base plate 40 of thelamination fixture (see FIG. 3). While two sets of packages per fixtureare shown assembled on the pins 15 in FIG. 3, it may be desirable toprovide a greater number of packages in each group, say from one packageto four packages. The packagesare separated by Teflon sheets 36 from thefianges of the pin 15, frornthe surfacerofjfthe' baseplate'40, vandifrom each otherto permit the packages 34 to be readily separated afterlamination. f i n The packages 34 themselves include a sheet104|of`vinyl material, which may be a filrn 0.020 in. thick. The circuit=board is next. Thisv boardV will be Yreferred to as the cover board. Acircuit layer 107 which provides aground plane is etched on thev outerside ofthe board. The other surface of the board has been etched free ofcopper, as was explained above. The next layerY is .a` sheet 109 ofnon-flowing B stage epoxy glass cloth which provides the bonding agentduring the lamination process'.`.This sheet is desirably a woven glassfiber' pre-impregnated with a non-flowing epoxy resin having a highinterflarninar bond` strength. Resin content may be from 65% by weightVto 55% by weight. Volatile content should be very low, say 0.5% byweight maximum.V It should'have, limit'ed'con,` trolled fiow (e.g., 1&4inch on edges) and a rapid gel time, say less than 30 seconds.Inasrnuchas this material hasno significant fiow, it does lnot givevrise to` shear Stresses caused by flow during the lamination process.This pre-v serves registration among the boards (viz. the coverboard 105will not slip laterally with respect to the l-bottom circuit board 110).This preserves the axial alignment of the drilled holes with the pads onthe printed circuits. Moreover, the non-flowing material when laminatedunder conditions of equalizing pressure provides ,ver'vpreci'se spacingbetween the circuit layers, as is necessary for Vstrip line circuitry.The circuit board 110 is copper cladon both sides. The inner side isetched while the outerside is-not etched. A circuit is provided4 on theouter, side, if re quired, in the latter stages of the process. Inasmuchaslpthis outer side is not etched, it provides good .electricallconrtinuity for the plating of drilled holes. However, the coppercladding 111, which as noted above is not etched, maybe etched in thestep 101 of the preparation of the board, if desired. The lowermostlayer'is a sheet 112 of vinyl phenolic glass cloth, say 0.004 in. inthickness. `It desirably has a resin content of 35% Vby weightanda.volatile content of 4% to 7.5% by weight. lThe vinyl sheet 104 overthe cover board and the vinyl phenolic glassl cloth sheet 112 on theunder side ofthe board serve asmasks to facilitate drilling andreaining0 quality "of the plated through holes and to prevent damage tothe boardsfduring etching, and also to prevent unwanted deposition ofelectrodeless copper on exposed Teflon during pre-plating, principallyin the cutout areas.

Referring to FIG. 3,'one fixture is-,shown after com pletion of the step114 of assembly. Lamination is done in a press; the platens 42 and 54ofwhich are-'shown -in FIG. 3. A bag 44 containing an ,elastometricV gumVmaterial 46, which ows within bag confines under heat and pressure, isdisposed between theplaten 42 and the fbottomgof the base plate 40. Thegum material may be ofthe type sold under the trade name Press-To-Flo.by the Bloomingdale- Rubber -Division of American .Cyanimid Company.Abovethe fiangesof the pinslS there areV placed three sheets 50 of vinylmaterial (the same` material as sheet 104). These sheets of vinyl film,say .020 inch thick, are placed over both sets of-packages prior, tolamination. A sheet 51 of vinyl fluoride film material, -which pre;vents adhesion of the vinyl` sheets ,50 to the platen 54, is provided.The gum material and the vinyl sheets 52.1iow under pressureand provideuniform conformal pressure during lamination. lt has been found that theuse `of the pressure equalizing means of the entrapped gum and the vinylsheets provided an improvement over conventional rigidY pressureapplication systems andlper'mits vlamination without voids and ofprecise bonding film thickness in spite of the fact that the B stageepoxy glass cloth 109 (FIG` 2)-*is nonowing, l'plus allowing the use oflower laminating pressures which reduce resultant laminationstressesfThe aluminum'bagj also" slows heat transferandprovidesadditionaltimefor fixturing and loading before closing thePress. 1 f

`'ilnrfthestep 115 of laminating, the platens are -firstheat`ed"to`about*325\ F. The press is`then closed under no load."=Whilethe press is so closed-(dwell time), therternperature of the" plate 40is measured, say With a surface pyrorneter.- When this edge temperaturereaches 200 F., pressure is* applied slowlyuntil Ait reaches about 200p.s.i. A loading time is desirably from 5 to 10 seconds. Pressure andheat is'applied for-40 min. The pressure is continued after the heat isturned onc and the heat is allowed toj fall to about 100 `F. Then thepressure is removed, and the fixturefis removed from'the press. Apost-cure step has been'found not to be necessary by virtue of the factthat lamination techniques impart a minimal laminate stress. The' nseofBstage material also permits lamnation to be controlled readily by meansof control of thextnre plate' 40 temperature-and avoids reliance on theskill of the operator to observe the llow and polymerization'-characteristics of the adhesive, which is the case 'when ilowablematerials are used. v

In the next step'116, the fixture is disassembled by peeling off thelvinyl 50 and vinyl fluoride sheets 51 from the top of the packages andknocking out the pins in an arbor press.lThe packages are separated bymeans of the Teon sheet spacers 36.

In the next step 117, the holes, which are to be plated through, aredrilled and some of the holes are reamed. Since the mask104-.becomes.visually transparent after lamination, drilling can be doneby eye (viz. without special. indexing). It has been found in accordancewith theV invention that'reaming with a cleaning material serving asalubricant prevents smear of the Teflon over the Vcircuitlayers(particularly the annular rings of copper in the holes) Which'wouldotherwise preclude good continuity to the circuits via the platedthrough holes. Drilling is followed with reaming, but only those holeswith internal connections are desirably reamed. Prior to each downstroke of the reaming tool a lubricant, preferably a pure soap such asIvory soap` sold by the Procter and Gamble Company has; beenfound-'especially suitable as a lubricant. The vinyl and vinyl phenolicsheets 104 and 112 insure that there will be no burrs formed duringdrilling or reaming and prevents tearing or other damage to the groundplane layers on the upper and bottom surfaces of the cover board 105 andthe bottom board 110, respectively. Desirably, the reamer is made oftungsten carbide material. Desirably, also the down stroke is from thevinyl sheet 104 towards the vinyl phenolic sheet 112 which serves as abacker and insures clean cutting of the copper material on the downstroke while preventing smear and gouging, providing high quality smoothholes with clean internal rings.

Etching of the plated through holes is accomplished in two steps 118 and119. In the first step 11:8, the Teon material is etched, while in thenext step, the epoxy is etched back. The etching steps. provide cleanplated through holes with exposed annular rings and whole surfaces whichaccept the deposition of an electrodeless copper conductive coating. TheTelion etch is a sodium naphthalate salt solution. The solution soldunder the name Tetra-Etch by W. L. Gore Company of Newark, Del., will besuitable. The epoxy etch is accomplished by utilizing an epoxy etchantof concentrated 98% sulphuric acid which is rinsed off in hot water. Itis desirable to strip the mask after the Teflon etch 1118 as sulphuricacid will clean the copper surface. This may be accomplished byimmersing the board in hot water, say 200 F., for a few minutes, andthereafter peeling olf the vinyl phenolic mask. The mask stripping stepalso serves the purpose of washing off the sodium naphthalate solution.

After etching, comes the step 121 of the deburring of any of the reamedholes in the-outer circuit cover surface of the layer 111 which mayappear. This may be done by a rounded glass stirring rod mounted in adrill.-This deburringv is generally vlimited to reamed holes only.

vAfter deburring, electrodeless kcopper is deposited in the next step122. This provides a conductive coating over the inner surfaces vof allof theV holes. The vinyl mask 104 prevents undesirable deposition ofconductive materialon exposed Tellon substrate areas,say in the areas132, 134 (FIG. 4).. The following step 123 is electrolytic plating, sayina copper pyrophosphate plating bath. Contact-may vbe madeto the groundplane layer 111 for good plating continuity. Inasmuch as removal of thevinyl mask 104 prior to plating could strip away some of theelectrodeless copper coating, it is desirable to stop the platingprocess after some copper has deposited and after the, copper hassuiicient strength to stand alone and then removing the vinyl mask. Thismay be accomplished merely by peeling the mask off the boards. After themask is removed, the boards are scrubbed thoroughly with a brush andscrubbing compound. The boards are then replaced into the baths andplating continues. Plating may take place, for a six inch by eight inchblank, at 9 amps current for 15 minutes prior to mask removal and for anadditional 45 minutes at 17 amps after the mask has been removed.

'Ihe next step 124 involves the application of the circuit pattern tothe layer 111 by means of silk screening on a suitable solder resist.After this step, a solder plating step is accomplished. In the next step126, the resist is stripped with a suitable solvent.

Next a spray coating of fast-drying acrylic resin is applied, but onlyto the top side (viz. over the ground plane layer 107). It should benoted that the edges of the board may be plated to provide an edgeshield, suitably when boards are dimensioned to size prior to plating.In other words, an electrodeless copper deposit is applied to the edgesin step 122 which is plated in step 123 and then solder plated in step12-5. Acrylic spray coating may cover the sides, as well as the surfaceof the layer 107.

In the next step 128, the bottom layer 111 is etched to provide thecircuit thereon. After the etchant is cleaned, the boards are cut tosize in step 129. The acrylic coating is then stripped in step byimmersing in a hydrocarbon solvent material, such as Freon TMC sold byE. I. du Pont de Nemours. The boards are then touched up and cleaned andare available for use.

FIGS. 4 and 5 illustrate a typical board after it is completed. Thecover board 105 and the bottom board 110 are illustrated. It will benoted how the B stage sheet 109 bonds these boards together. The cutouts132 and the hole 134 which are precut in the boards are alsoillustrated. Note also the notch 136 which is precut in the lower board110. The circuits 138 and 139 on the exterior surfaces of the boards 105and 110 are substantially continuous and provide shield and groundplanes so that the circuit board is operable as a strip line. Connectionmay be made for transmission of microwave energy to the boards. Forexample, a microwave tube may be connected to the extensions 140 and 142of the inner cir cuit provided on the uppermost side of the lower board110. An element such as a choke 144 may be connected via the cutouts andhole 132 and 134. Connections may also be made to the circuits via thecutouts.

From the foregoing description it will be apparent that there has beenprovided an improved stripline multi-layer printed circuit board whichis provided from circuit boards having Teon substrates. While apreferred process and circuit board configuration has been described, itwill be appreciated that variations and modifications thereof within thescope of the invention or utilizing the 'features thereof willundoubtedly suggest themselves to those skilled in the art. Accordingly,the foregoing des'cription should be" taken merely as illustrative andnotin anylimitig sense.v J1 y 1 What-isclaimedis: i

1.l A method of making a multi-layer strip lineprinted circuitboa`1 'dusing afpair of -boards havingTefion 'substrates,"eachv ofwhich boardsvis clad with conductive material on both; surfaces Athereof whichcomprises the steps 4removing the conductive material 'from the innersur- -face of the first of said pair of boards, 'y

providing circuits on theinner surface of the second' of -said pairr ofyboards then laminating the inner surfaces of said first and secondboards together with a sheet of substantially non-flowing B stage epoxyglass cloth therebetween,

drilling holes through said laminated boards, and

making plated connections to said circuits and said cladding via saidholes.

Z. The invention as set forth in claim 1 including the steps of applyingmasks of vinyl material on the outer surfaces of said rst and secondboards prior to said laminating step and then laminating said vinylmask'material to said outer surfaces during said laminating step, andremoving said masks after said step of making platedthrough connections.

3. The invention set forth in claim 2 wherein one of said masks is vinyland the other is vinylphenolic whereby the masks are selectivelyremovable, the vinyl mask being transparent after lamination anddrillable, thereby facilitating hole positioning during drilling andpreventing deposition of conductive material on the outer surface of oneof the laminated boards.

4. The invention set forth in claim 3 wherein said holes are drilled soas to intersect two dissimilar materials (Teon and epoxy), and includingthe step of fabricating openings in one of said boards and correspondingopenings in said epoxy glas-s cloth prior to lamination whereby toprovide access to circuits internal of the multilayer board formed afterlamination.

5. The method of making a multi-layer printed circuit board from aplurality of circuit boards, each having a Teon substrate comprising thesteps of laminating said boards together with a layer of sub- 1drillingy holesfthroughi said lidar ,Y Y .agr circuits thereon, said'drillinggbeingaccqmp vetching said. holes-to provide,` clean annul ingstep is adapted to apply .a 4copper-,plating over the conductivematerial deposited, on: said .board, and. -incluchl-V ing the step ofprinting an image which'will accept pppex.y plating on at least one sideof saidboard` and thenr solder plating over said image. f

. stantially non-owing-VBstageepozryglass loth th between, l

with the aid of a lubricant,

g the ,circuit layers intersect said holes, ",saiclw tch'ingl.` beingaccomplishedin aiirst step of immersing said boards-in asolution whichis adaptedgto etchzthe. Teflon material, andy then, immersinglsaidvbQarlsJl! asolution which gis adapted to etchv the epoxy glaSfSl,

material, f, f i l' depositing a coating of conductivematerial onboards, and f a thereafter plating the,v dissimilarA composite materialsconstituting said laminated boards electrolytically;yy t 6. Theinvention set forth in claim Sawherein ,said-.pl

7. The invention set forthrin `claimY,v6,.'including."the

steps of applying a coating of yacrylic resinvmatejrial :to the surfaceof said board which is not solder ;plated, immersing said solder platedsurface in a Copperetchant so as to remove the copper inthe-areawhichfisnot solder plated, thereby to-provide Va solderf plated; circuitover a surface of said board. j

JOHN H. MACK, Primary .Examinerff Y T. TUFARIELLO, Assisemt,Exaiiiinel

